For me quality ambient is something I can put on for background music for hours and/or I can take a deep listen sessions with headphones. Some artists and records that has survived the test of time for me are:
Brian Eno
Obvious choice yep, but after listening ambient so many years and checking out lot of different artists and styles, Eno still rules big time. Albums have quality production, sound and compositions are perfect. Another thursday afternoon, Lux, Ambient 2 and 4 are maybe the most listened ambient albums ever for me and I still return to them when I want to listen something but don't know what to put on. Reflection is also good stuff. Also a big fan of what he has done with Robert Fripp and Cluster etc, but those are the different story. I like pretty much almost everything he has done in ambient field.
Steve Roach
I have most number of Steve Roach albums from ambient genre in my collection. Enough variation and something for every mood in his discography. Spacey "darker" stuff for night time listening when going to sleep and nice soothing background muzak for daydreaming. I still prefer the droning sound type of stuff (Mystic chords & sacred places, Immersions, deeper silence etc.) and minimal compositions; Structure from silence and quiet music etc. His "berlin school" style albums I don't care about at all...
Biosphere
Huge fan! Geir Jenssen is the master of loops in ambient. Maybe the nordic melancholy in his best albums resonates deeply in me, but I always return to Biosphere and I believe that this is the most listened artist for me during last 15 years. Shenzou, Substrata, Cirque and Dropsonde are masterpieces. I can put each of those albums for repeat to my cd player and never get bored. Works also in headphone listening sessions. Atmospheric stuff with good ear for details and minimalism done right. Autour de la lune is great drone album and N-plants ambient techno also does the trick to me.
Matthias Grassow
Same thing as Steve Roach for me; I listen to Grassow a lot and there are album for almost every occasion. Quality ambient and not only synth stuff. Also some concrete sounds like singing bowls, sitars, choir chants in some albums. Not used in a cheesy new-age way.
Gas / Wolfgang Voigt
Ah! The master of scarf ambient/techno! I like everything Gas has made. Hazy atmoshperes that keeps your attention in headphone sessions or you can just the albums for background. Rückverzauberung 10 / Nationalpark under his own name is reeeeeally good ambient. I remember the first listen I tried it via spotify and was just meh, but I had already purchased it on cd. Forgot the cd for a while and when I returned to it and listened it with stereo equipment I realised how good it is. Modern minimal classical style, greatly composed and sound is super good!
Dark ambient and drone are different genres for me so I don't feel like listing the usual names so I return to those in another topics.
From noise side of "ambient" I like Mike Shiflet and Damien Romero.